Digital image forming apparatus

ABSTRACT

In a digital image forming apparatus, a document image is read as digital data, and an image type of bi-level image or multi-level image is discriminated from the digital data. For a bi-level image, an intensity of laser beam for exposing a photoconductor is modulated according to the image data with a duty ratio of 100%. On the other hand, for a multi-level image such as a photograph image, the intensity of laser beam is modulated with a duty ratio in the unit of two or more dots according to the image data. The duty ratio and the laser output power are changed according to a type or document image or according to sensitivity characteristic of the photoconductor.

RELATED APPLICATION

This is a divisional of allowed application Ser. No. 08/365,507, filedDec. 28, 1994, now U.S. Pat. No. 5,729,626.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital image forming apparatus suchas a digital printer or a digital copying machine which expressesgradation by modulating light intensity.

2. Description of the Prior Art

In a digital image forming apparatus such as a digital printer or adigital copying machine which forms an image by modulating a lightintensity of a laser beam, a photoconductor is exposed in raster scanwith a laser beam at an intensity modulated according to image data of adocument read by an image reader. In the raster scan, the photoconductoris exposed during a time in correspondence to a length of one dot. Thus,each dot is formed in correspondence to an amplitude of image densitydata, and an electrostatic latent image is formed on the photoconductor.

The modulation of laser intensity can form an image with a highresolution, while it can form an image with smooth gradationcharacteristic. However, this method has a disadvantage in that pitnoises along the subscan direction is liable to occur to deteriorateimage quality. Further, gradation correction needed to correct thenonlinear gradation characteristic between image density on a documentand output image density on a paper is affected by environmentconditions and the like. Especially for a color image forming apparatus,image quality and gradation correction are important because a documentincluding a color photograph is reproduced usually.

In order to solve the problems, U.S. patent application Ser. No.07/971,055, now U.S. Pat. No. 5,420,614 discloses that an interval isprovided between light emitting periods or the laser beam irradiates thephotoconductor intermittently. By providing such intervals, theabove-mentioned pitch noises do not become noticeable and this improvesthe smoothness of an image. Further, the nonlinearity of gradationcharacteristic which also affects image quality is also improved, andthe image can be reproduced more stably. A duty ratio, defined as aratio of a light-emitting period to a sum of the light-emitting periodand a non-light-emitting period, can be changed. It is also disclosedthat the above-mentioned interval is provided in the unit of a pluralityof dots, say two dots.

Though the introduction of duty ratio improves the image quality andgradation characteristic as mentioned above, the image quality alsodepends on a kind of image such as a photograph image, and gradationcharacteristic is affected by environment such as humidity. Therefore,it is desirable that an image quality is improved more by taking theminto account.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an digital imageforming apparatus which forms an image of good image quality.

Another object of the present invention is to provide an digital imageforming apparatus which reproduces an image with a more linear gradationcharacteristic.

In one aspect of a digital image forming apparatus, a surface of aphotoconductor is scanned to be exposed with a beam in raster scan so asto form an electrostatic latent image thereon, and an intensity of thebeam is controlled in accordance to image data detected on a document.In the exposure, it is inhibited to expose the photoconductor everypredetermined period, so that a time interval is formed betweenlight-emitting periods. On the other hand, a type of an image such as acharacter image or a photograph image is discriminated, and a ratio ofthe light-emitting period to the interval (or a duty ratio defined as aratio of a light-emitting period to a sum of the light-emitting periodand a non-light-emitting period) is changed in accordance to thediscriminated type. The type may be discriminated in various portions ofa document. In the exposure, an interval may be provided in the unit ofa plurality of dots. Preferably, the intensity of the beam is alsochanged according to the ratio. A plurality of combinations of thenumber of dots and the ratio may be determined for various image types.The ratio may be changed according to the sensitivity of thephotoconductor which is affected for example by humidity.

An advantage of the present invention is that an image can be reproducedwith good image quality.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings, and in which:

FIG. 1 is a schematic view of a digital color copying machine;

FIG. 2 is a block diagram of image data processor;

FIG. 3 is a histogram of brightness V of pixels in a processing block of100*100 pixels for a character image;

FIG. 4 is a histogram of brightness v of pixels in a processing block of100*100 pixels for a photograph image;

FIG. 5 is a histogram for discriminating a character image;

FIG. 6 is a diagram for illustrating a digital image data of M*N pixelswith reference to a processing block B_(X),Y of m*n pixels;

FIG. 7 is a flowchart of image discrimination;

FIG. 8 is a flowchart of histogram processing;

FIG. 9 is a flowchart of block discrimination processing;

FIGS. 10A and 10B are diagrams of differential filters;

FIG. 11 is a graph of a distribution of differentials (absolute values)for image discrimination;

FIG. 12 is a block diagram of a print head;

FIG. 13 is a timing chart of density level, average, light emissionsignal and driving current;

FIG. 14 is a graph of photoconductor characteristic in environments of(a) high temperature and high humidity, (b) room temperature andordinary humidity, and (c) low temperature and low humidity; and

FIG. 15 is a graph of surface potential and a change in duty ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views, adigital copying machine of an embodiment of the invention is explainedbelow which forms an image on a photoconductor by modulating lightintensity of a laser beam in raster scan to form an electrostatic latentimage on the photoconductor. In the laster scan, the photoconductor isexposed during a predetermined period of time in correspondence to oneor more pixels. Thus, a dot is formed in correspondence to an intensityof image density data. An appropriate duty ratio, defined as a ratio ofa light-emitting period to a sum of the light-emitting period and anon-light-emitting period, is set according to density level of documentimage.

FIG. 1 shows the digital color copying machine schematically, and itconsists of an image reader 100 and a printer 200 for reproducing animage read by the image reader 100. In the image reader 100, a scanner10 has an exposure lamp 12 for illuminating a document placed on aplaten 15, a rod lens array 13 for collecting light reflected from thedocument, and a CCD color image sensor 14 for converting the collectedlight to an electric signal. When the document is read, the scanner isdriven by a motor 11 to move along a direction shown with an arrow toscan an image on the document. An image on the document illuminated bythe exposure lamp 12 is converted by the image sensor 14 to electricsignals of red, green and blue. A read signal processor 20 processes theelectric signals to generate 8-bit gradation data of yellow, magenta,cyan or black, while it also make a histogram on the gradation data todiscriminate image type of bi-level image or multi-level image in theunit of a processing block, as will be explained later.

In the printer 200, a control unit 30 receives 8-bit gradation data anddiscrimination signals from the read signal processor 20, and itcorrects the gradation data according to gradation correctioncharacteristic based on various data from sensors and supplies drivesignals to a print head 31 for modulating a laser beam by using the dutyratio and the standard laser output power determined according to thediscrimination signal and the environment factor such as humidity ortemperature, as will be explained later. A rotatable photoconductor drum41 is illuminated by an eraser lamp 42 and is charged uniformly by asensitizing charger 43 before each copy operation. A laser beam emittedby the print head 31 is reflected by a mirror 39 to expose thephotoconductor drum 41 to form an electrostatic latent image. One ofdeveloping units 45a, 45b, 45c and 45d for cyan, magenta, yellow andblack is selected to develop the latent image to form a toner image. Thetoner image is transferred by a transfer charger 46 onto a paper on atransfer drum 51. This scan and print process is repeated four times forthe four colors. Then, the paper is separated by operating a claw 47 andfixed by a fixing unit 48 to be discharged onto a tray 49.

FIG. 2 shows a block diagram of the image processor 20. Electric signalsof a document image received from the CCD sensor 14 is converted by anA/D converter 21 to digital multi-level (8-bit) values of red, green andblue. The digital value is corrected by shading correction circuit 22for shading correction, and the corrected value is stored in a memory23.

Data stored in the memory 23 represent an intensity of light reflectedfrom the document, and they are converted by a log converter 24 todensity data. Next, an undercolor-remove/black-paint circuit 25 removesa black component from the data of red, green and blue and generatesblack data. The resultant data of red, green and blue are converted by amasking circuit to density data of cyan, magenta or yellow, and thedensity data is multiplied with a coefficient for correction by adensity correction circuit 27. Then, the density data is corrected onspatial frequency by a spacial frequency correction circuit 28 and sentto the control unit 30.

On the other hand, the data after shading correction is also sent to aV-value calculator 60 which calculates a brightness V for each pixel.(Usually, brightness is represented as Y, but it is represented as V inthis specification.) A histogram circuit 61 makes a histogram ofbrightness V in the unit of a processing block made of a prescribedpixel matrix, as will be explained later in detail. A decision circuitdecides a document type according to the histogram generated by thehistogram circuit 61. A decision result is once stored in a memory 63,and it is also sent to the control unit 30.

Next, discrimination of image type of bi-level image or multi-levelimage by the image processor 20 is explained. A bi-level image is animage such as a character image which consists mainly of black and whitelevels. On the other hand, a multi-level image is an image whichincludes various density levels and it includes a photograph image. FIG.3 shows an example of a histogram of brightness V obtained by theV-value calculator 60 of pixels in a processing block of 100*100 pixelsfor a character image. Because a character image is a bi-level image,the frequency is high only near zero and near a maximum level. On theother hand, FIG. 4 shows an example of a histogram of brightness V ofpixels in a processing block of 100*100 pixels for a photograph image.Because a photograph image is a multi-level image, the frequencyscatters over a wide range. It is clear that a histogram for aphotograph image is different clearly from that for a character image,and the discrimination circuit 62 decides the document type such ascharacter image or photograph image by discriminating features of ahistogram. That is, if a histogram of an image is included in a hatchedregion shown in FIG. 5, the image is decided as a character image. Thehatched area does not exist at intermediate brightness V.

Next, image discrimination using a histogram by the histogram circuit 61and the discrimination circuit 62 is explained. As shown in FIG. 6,digital image data of M*N pixels are divided into processing blocksB_(X),Y each consisting of m*n pixels, where m and n are divisors of Mand N. Therefore, (M/m)*(N/n) processing blocks are included in theimage. A frequency of brightness V=i (=1-255) in a processing blockB_(X),Y is denoted as Z(X, Y, i).

FIG. 7 shows a main flow of image discrimination. First, variables areinitialized (step S1), or i, Y and y are set as zero. In this flow, xdenotes an x-th pixel along main scan direction while y denotes an y-thpixel along subscan direction in a processing block B_(X),Y (refer toFIG. 6). Next, Y is set by increasing it by 1 (step S2). Next,histograms of processing blocks B₁,Y to B_(M/m),Y are calculated. Thatis, y is set by increasing it by 1 (step S3), and histograms ofprocessing blocks B_(X),Y (X=1-M/m) are calculated as will be explainedlater (step S4, refer FIG. 8). The flow returns to step S3 if y isdecided not to be equal to n or a number of dots along subscan direction(NO at step S5). This calculation is repeated until y is decided to beequal to n (YES at step S5).

Then, it is discriminated if images in the processing blocks B₁,Y toB_(M/m),Y are a bi-level image or a multi-level image (step S6, referFIG. 9), and discrimination signals S for the processing blocks are sentto the memory 63.

Then, the flow returns to step S2 for other processing blocks if Y isdecided not to be equal to N/n (No at step S8), and this calculation isrepeated until Y is decided to be equal to N/n (YES at step S8) orcalculation on all processing blocks is completed.

FIG. 8 shows a flow of histogram processing (step S4 in FIG. 7) formaking histograms of processing blocks B_(X),Y (X=1-M/m). First, X isset as zero for initialization (step S11). Next, X is set by increasingit by 1 (step S12). Then, x is set as zero for initialization (stepS13). Next, a histogram of a processing block B_(X),Y in y-th line iscalculated. That is, x is set by increasing it by 1 (step S14), while aV value for a pixel at a position (x, y) is set as i (step S15). Then, afrequency Z(X,Y,i) of brightness i in the processing block B_(X),Y isincreased by one (step S16). Then, the flow returns to step S14 if x isdecided not to be equal to m (NO at step S17), and this calculation isrepeated until x is decided to be equal to m (YES at step S17) or untilthe calculation of the histogram in the processing block B_(X),Y iscompleted.

If the histogram in the processing block B_(X),Y is completed (YES atstep S18), it is decided next if X is equal to M/m (step S18). The flowreturns to step S12 if X is decided not to be equal to M/m (NO at stepS18), and this calculation is repeated for another processing blockuntil X is decided to be equal to M/m (YES at step S18) or until shecalculation of the histograms for the processing blocks B_(X),Y(X=1-M/m) are completed.

FIG. 9 shows a flow of block discrimination processing (step S6 in FIG.7) for supplying discrimination signals S which show that images in theprocessing blocks B₁,Y to B_(M/m),Y are a bi-level image or amulti-level image. First, variables are initialized (step S21), or i, Xand a coefficient j(X,Y) are set as zero. Next, X is set by increasingit by 1 (step S22). Next, the coefficient j(X,Y) is calculated. That is,i is set first by increasing it by one (step S23), and if the frequencyZ(X,Y,i) is not smaller than a reference value f(i) (YES at step S24),the coefficient j(X,Y) is increased by one (step S25). The referencevalues f(i) are shown in FIG. 5 as a boundary of the hatched area. Theflow returns to step S23 if j is decided not to be equal to 255 (NO atstep S26). In other words, this calculation is repeated until j becomes255 (YES at step S26) or coefficients for all i is determined.

Then, a discrimination signal S(X,Y) for the processing block B_(X),Y isdetermined. That is, if the coefficient j(X,Y) is equal to or less thana threshold value J (YES at step S27), a discrimination signal S(X,Y) isset as zero (step S27) or the image in the block is discriminated as abi-level image, otherwise it is set as one (step S28) or the image inthe block is discriminated as a multi-level image.

Then, the flow returns to step S22 if X is decided not to be equal toM/m (NO at step S30) for other processing blocks, and this calculationis repeated until X is decided to be equal to M/m (YES at step S30) oruntil the discrimination signals S(X,Y) on all the processing blocksB₁,Y -B_(M/m),Y are obtained.

The discrimination of image type is not limited to the procedureexplained above. For example, the brightness data or the density datamay be processed with a differential filter and the image type may bediscriminated according to the distribution of the obtained differentialvalues. FIGS. 10A and 10B show examples of differential filters fordetecting an edge of an image along main scan direction and along asubscan direction, respectively. Both filters may be used fordiscriminating the image type. In this case, an average of a sum of theabsolute values of differential values obtained with use of the twofilters is used for the discrimination. For example, as shown in FIG.11, if an average of differentials (absolute values) exists within ahatched area, the image is discriminated as a bi-level image, otherwiseit is discriminated as a multi-level image. This discrimination is basedon a fact that a bi-level image has a peak in the hatched area exceedinga threshold level say 10% in the hatched area in FIG. 11 except nearzero.

FIG. 12 shows a block diagram of the control unit 30. The 8-bit imagedata are received from the image signal processor 20 by the interface 80to be stored in a first-in first-out (FIFO) memory 81. Thediscrimination signals S are stored in another first-in first-out memory82. A printer controller 201 reads an appropriate gamma correction tablefrom the gamma data ROM 94 according to the measured values of atemperature sensor 95, a humidity sensor 96, a surface potential sensor97, and other sensors such as AIDC sensor not shown explicitly. Then, itsends the gamma correction table to a gamma correction section 83 and astandard laser output power in correspondence to the selected gammacorrection table to the gain change signal generator 93. The gammacorrection section 83 corrects the gradation data read from the FIFOmemory 81 by using the gamma correction table. If the discriminationsignal is zero or the image type is photograph image, the gammacorrection section 83 processes the correction in the unit of twopixels. That is, the density level data of two neighboring pixels areaveraged, and the gradation correction is performed on the average. Onthe other hand, if the image type is bi-level image, the light-emittingperiod is set as one dot. The corrected gradation data is sent to a D/Aconverter 84. The D/A converter 84 converts the corrected gradation datato an analog voltage, which is amplified by an amplifier 85 according toa gain or output power set by the change signal generator 93 by usingswitches SW1, SW2 and the like. The gain of the amplifier 85 isdetermined by the printer controller 201 according to the discriminationsignal received from the FIFO memory 82 and measured values of thehumidity sensor 96 and the temperature sensor 95 and sent to the gainchange signal generator 93. The amplified analog voltage is sent througha drive I/O circuit 86 to a driver 87 for a laser diode 88 which emits alight at an intensity modulated according to image data. On the otherhand, the light emission signal generator 90 sets a duty ratio for thedensity level according to the duty ratio received from the printercontroller 201. That is, the light emission signal generator 90 sends alight emission signal only in a light emission period through a parallelI/O 89 to the laser diode driver 87.

If the image type is discriminated as bi-level image, the duty ratio isset as 100%. On the other hand, if the image type is discriminated asmulti-level image, the duty ratio is set between 70-95% depending on theenvironment factors. Then, the laser diode driver 87 generates a drivingcurrent for the laser diode 88 only when the light emission signal isreceived. FIG. 13 shows an example of duty ratio setting for sequentialsix pixels. As explained above, the gamma correction section 83processes the correction in the unit of two pixels. That is, the densitylevel data of two neighboring pixels are averaged, and the gradationcorrection is performed on the average. The light emission signal havinga duty ratio is also handled in the unit of two dots for multi-levelimage. When density levels of sequential six pixels shown in FIG. 13 arereceived, averages thereof are calculated, and a driving current isgenerated by using a duty ratio of say 80% depending on the environmentfactors. The driving current is enhanced to compensate the decrease induty ratio.

Next, gradation correction determined by the printer controller 201 isexplained. The sensitivity of the photoconductor for the exposure by thelaser beam is affected by environment factors of humidity andtemperature. FIG. 14 shows a photoconductor characteristic inenvironments of (a) high temperature and high humidity, (b) roomtemperature and intermediate humidity and (c) low temperature and lowhumidity. The sensitivity or the decrease in surface potential due toexposure at a brightness level decreases with decreasing humidity andwith decreasing temperature.

FIG. 15 shows surface potential and an effect of duty ratio thereon. Arectangular wave shown at the top in FIG. 15 represents an intensity ofexposure light on the photoconductor along the main scan direction forthree dots. There are provided an interval between light-emittingperiods. A solid line and a dashed line correspond to duty ratios of 50%and 67%, respectively, where duty ratio denotes a ratio of alight-emitting period to a sum of the light-emitting period and anon-light-emitting period. The intensity of the rectangular wave forduty ratio of 50% is higher than that for duty ratio of 67% in order tocompensate the decrease in total quantity of exposure light per dot. Thesurface potential V_(o) before exposure decreases to V_(i) afterexposure at the maximum density level. A maximum attenuation potentialor residual potential V_(r) represents a lowest surface potential causedby exposure. Toners adhere to portions where the surface potential V_(i)is lower than the development bias voltage V_(B) for the developmentunit 45a-45d.

The surface potential decreases exponentially with the density level orexposure intensity generally. Therefore, if the exposure intensity isincreased when the sensitivity decreases, the attenuation characteristiccan be maintained and the gradation curve is not changed. However, whenthe sensitivity decrease, the maximum attenuation potential V_(r)increases or in FIG. 15 it increases from V_(r) to V_(r) '. Therefore,the attenuation potential saturates before the intensity is increased toa maximum. Then, as shown in the curve "a" in an environment of highhumidity and high temperature in FIG. 14, the surface potential does notdecrease exponentially. Then, if the intensity is increased forcorrection, the attenuation becomes larger at highlight levels while itbecomes smaller at high density levels, and this worsens the gradationcharacteristic. Then, in the example, the duty ratio is increased forfrom 50% to 67% while decreasing the intensity from say 1.00 mW to 0.80mW, so that the attenuation potential at the maximum density level V_(i)becomes lower than the maximum attenuation potential V_(r) '. In otherwords, the decrease in intensity of laser power is compensated byincreasing duty ratio. Thus, the photoconductor characteristic is keptthe same even in bad environment.

If the discrimination signal S(X,Y) for a processing block B_(X),Y isequal to one or if the image type is discriminated as bi-level image,the printer controller 201 sets the light emitting period in the unit ofa dot, the duty ratio of 100% and a standard laser output power of 1.00mW, because the resolution is emphasized. On the other hand, if thediscrimination signal S(X,Y) is equal to zero or if the image type ismulti-level image, the printer controller 201 sets the light emittingperiod in the unit of two dots, and it also sets the duty ratio and thestandard laser output power depending on environment factors such ashumidity and temperature, as shown in Table 1 or 2. If the surfacepotential is measured instead of the humidity and temperature, the dutyratio and the standard laser output power are changed according to Table3. For a multi-level image, in the gamma correction circuit 83, thedensity level data of two pixels are averaged, and the gradationcorrection is performed on the average.

                  TABLE 1                                                         ______________________________________                                        Duty ratio and standard laser output power in various humidity ranges         Humidity range                                                                              Duty ratio                                                                             Standard laser                                         (% RH)        (%)      output power (mW)                                      ______________________________________                                         0-20         70       1.40                                                   21-40         75       1.30                                                   41-60         75       1.30                                                   61-80         80       1.32                                                    81-100       90       1.08                                                   ______________________________________                                    

If temperature is also considered in addition to humidity around thephotoconductor, duty ratio (%) and standard laser output power (mW) areset in correspondence to the humidity and temperature according to Table2. For example, if the humidity is between 0 and 35% RH and thetemperature is between 0 and 12° C., the duty ratio is set at 75% andthe standard laser output power is set at 1.48 mW.

                  TABLE 2                                                         ______________________________________                                        Duty ratio (%) and standard laser output power (mW) depending                 on humidity (% RH) and temperature (° C.)                              0-35        36-60      61-80      81-100                                      (% RH)      (% RH)     (% RH)     (% RH)                                      ______________________________________                                         0-12 75%       75%        80%      90%                                       (° C.)                                                                       (1.48 mW) (1.48 mW)  (1.39 mW)                                                                              (1.18 mW)                                 12-19 75%       75%        80%      90%                                       (° C.)                                                                       (1.39 mW) (1.39 mW)  (1.30 mW)                                                                              (1.18 mW)                                 19-26 75%       75%        80%      90%                                       (° C.)                                                                       (1.30 mW) (1.30 mW)  (1.21 mW)                                                                              (1.09 mW)                                 26-33 75%       75%        80%      90%                                       (° C.)                                                                       (1.21 mW) (1.21 mW)  (1.12 mW)                                                                              (1.00 mW)                                 34-40 75%       75%        80%      90%                                       (° C.)                                                                       (1.12 mW) (1.12 mW)  (1.06 mW)                                                                              (0.94%)                                   40-   75%       75%        80%      90%                                       (° C.)                                                                       (1.03 mW) (1.03 mW)  (1.00 mW)                                                                              (0.88 mW)                                 ______________________________________                                    

Further, the duty ratio and the standard laser output power may bedetermined by detecting the sensitivity change of the photoconductor andthe maximum attenuation potential by measuring the surface potential onthe photoconductor, without considering environment factors such ashumidity and temperature. For example, a surface potential sensor 97 isprovided near the photoconductor drum, as shown in FIG. 1, and thesurface potential is measured when the photoconductor is irradiated witha laser beam of a prescribed intensity. Then, the duty ratio and thestandard laser output power are determined according to the measuredvalue.

Further, in order to improve the precision of setting the duty ratio andthe standard laser output power, they may be determined according toTable 3 by measuring the surface potential on the photoconductor at alow density level and at a high density level. In Table 3, A(V)represents a surface potential of the photoconductor when thephotoconductor is sensitized at 950 V and exposed at a lower laseroutput power of 0.15 mW, while B(V) represents a surface potential whenthe photoconductor is sensitized at 950 V and exposed at a higher laseroutput power of 1.50 mW.

                                      TABLE 3                                     __________________________________________________________________________    Duty ratio and standard laser output power                                    B [v]                                                                         A [v] 0˜29                                                                           30˜59                                                                          60˜89                                                                          90˜119                                                                         120˜149                                                                        150˜179                                                                        180˜209                                                                        210˜             __________________________________________________________________________    650˜669                                                                       60%(1.48 mW)                                                                         65%(1.39 mW)                                                                         70%(1.33 mW)                                                                         75%(1.24 mW)                                                                         80%(1.15 mW)                                                                         85%(1.09 mW)                                                                         90%(1.03                                                                             100%(0.91 mW)          670˜689                                                                       60%(1.54 mW)                                                                         65%(1.47 mW)                                                                         70%(1.39 mW)                                                                         75%(1.30 mW)                                                                         80%(1.21 mW)                                                                         85%(1.15 mW)                                                                         90%(1.04                                                                             100%(0.97 mW)          690˜709                                                                       60%(1.63 mW)                                                                         65%(1.54 mW)                                                                         70%(1.45 mW)                                                                         75%(1.36 mW)                                                                         80%(1.27 mW)                                                                         85%(1.21 mW)                                                                         90%(1.12                                                                             100%(1.03 mW)          710˜729                                                                       60%(1.72 mW)                                                                         65%(1.60 mW)                                                                         70%(1.51 mW)                                                                         75%(1.42 mW)                                                                         80%(1.33 mW)                                                                         85%(1.27 mW)                                                                         90%(1.18                                                                             100%(1.06 mW)          730˜749                                                                       65%(1.72 mW)                                                                         65%(1.69 mW)                                                                         70%(1.57 mW)                                                                         75%(1.48 mW)                                                                         80%(1.39 mW)                                                                         85%(1.33 mW)                                                                         90%(1.24                                                                             100%(1.09 mW)          750˜769                                                                       65%(1.75 mW)                                                                         65%(1.72 mW)                                                                         70%(1.63 mW)                                                                         75%(1.54 mW)                                                                         80%(1.45 mW)                                                                         85%(1.39 mW)                                                                         90%(1.27                                                                             100%(1.15 mW)          770˜789                                                                       70%(1.75 mW)                                                                         70%(1.72 mW)                                                                         70%(1.69 mW)                                                                         75%(1.60 mW)                                                                         80%(1.51 mW)                                                                         85%(1.45 mW)                                                                         90%(1.33                                                                              95%(1.27 mW)          790˜809                                                                       70%(1.78 mW)                                                                         70%(1.78 mW)                                                                         70%(1.75 mW)                                                                         75%(1.66 mW)                                                                         80%(1.57 mW)                                                                         85%(1.48 mW)                                                                         85%(1.45                                                                              95%(1.33 mW)          810˜829                                                                       75%(1.78 mW)                                                                         75%(1.78 mW)                                                                         75%(1.75 mW)                                                                         75%(1.72 mW)                                                                         80%(1.63 mW)                                                                         85%(1.54 mW)                                                                         85%(1.51                                                                              90%(1.42 mW)          830˜850                                                                       80%(1.78 mW)                                                                         80%(1.78 mW)                                                                         89%(1.78 mW)                                                                         75%(1.78 mW)                                                                         80%(1.69 mW)                                                                         80%(1.66 mW)                                                                         85%(1.57                                                                              80%(1.48              __________________________________________________________________________                                                           mW)                     NB: Duty ratio is represented in the unit of %, while the standard laser      output power in parentheses is represented in the unit of mW.            

As explained above, the digital copying machine discriminates a documentimage as a character image or a half-tone image. For a character image,the intensity of the laser beam is modulated with light emitting periodof one dot and duty ratio of 100%, to form an image of high resolution.Then, an image of high resolution can be formed. For a half-tone image,the light emitting period and the duty ratio are changed according toenvironment. Then, an image of better image quality is formed byeliminating pitch noises along subscan direction, and the gradationcharacteristic is improved to form a better and stable image.

In the embodiment, document image types are classified into bi-levelimage and multi-level image. However, in a modified embodiment, themulti-level image is further classified into a photograph image and adot image, and duty ratio may be changed between the photograph imageand the dot image. For example, the duty ratio may be set as 80% for aphotograph image and as 90% for a dot image, while it is set as 100% fora bi-level image.

In the embodiment, image type is discriminated simultaneously as andocument image is read. However, image type may be discriminated byusing document image data obtained in a prescan, and image data forprinting may be read after the image discrimination is completed.

Further, the discrimination may be carried out on the entire imageinstead of discriminating image type for each processing block.

A histogram may be made only on image data of one color among red, greenand blue. It is also possible to discriminate image type by using a sumof frequencies in ranges between 0 and 7, between 8 and 15, . . . ,between 240 and 247 and between 248 and 255. It is also possible to usedensity data instead of brightness data for image discrimination

In a modified example, the charge amount by the sensitizing charger 43is corrected to an appropriate value by measuring the surface potentialof the photoconductor at a portion not exposed before measuring thesurface potential at a portion exposed at a predetermined intensity.

In a different example, the duty ratio and the standard laser outputpower are determined according to photosensitive characteristic of thephotoconductor which is obtained by measuring the surface potential forportions illuminated at various intensities.

The duty ratio and the standard laser output power can be correctedbetter by taking into account both environment data and measured surfacepotentials.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. A digital image forming apparatus comprising:animage sensor which receives light from an original image and outputssignals with respect to the original image; a photoconductor; anexposure means for exposing a surface of said photoconductor to lightemitted from a light source so as to form an electrostatic latent imagethereon; an exposure control means for controlling an intensity of thelight of said exposure means according to the signals outputted by saidimage sensor; a light-emission control means for inhibiting saidexposure means to expose said photoconductor every predetermined periodso that a non-light-emitting time interval is formed betweenlight-emitting periods wherein said exposure means exposes the surfaceof said photoconductor; a discriminating means for determining an imagetype of the original image; and a ratio changing means for changing aratio of the light-emitting period to the interval in accordance to theimage type determined by said discriminating means.
 2. The digital imageforming apparatus according to claim 1, wherein said discriminatingmeans comprises a distribution detection means for detecting adistribution of image data for at least a portion of said originalimage.
 3. The digital image forming apparatus according to claim 1,wherein said discriminating means comprises a distribution detectionmeans for obtaining a frequency distribution on brightness data derivedfrom the image data for at least a portion of said original image. 4.The digital image forming apparatus according to claim 1, wherein imagetypes which can be determined by said discriminating means include abi-level image and a multi-level image.
 5. The digital image formingapparatus according to claim 1, wherein said discriminating meansincludes a distribution detection means which detects a distribution ofimage data for each of a plurality of regions of the original image anddetermines the image type for each of the plurality of regions.
 6. Thedigital image forming apparatus according to claim 1, wherein saiddiscriminating means determines an image type for each of a plurality ofregions of the original image.
 7. The digital image forming apparatusaccording to claim 1, wherein said ratio changing means increases theintensity of the light in accordance to the ratio.
 8. The digital imageforming apparatus according to claim 1, wherein said light-emissioncontrol means inhibits said exposure means to expose said photoconductorevery predetermined period corresponding to a period for exposing anumber of dots of image data, said number of dots being determined inaccordance to the image type determined by said discriminating means. 9.The digital image forming apparatus according to claim 8, furthercomprising a memory for storing a table of a plurality of combinationsof the number of the dots and the ratio related to the image typedetermined by said discriminating means.
 10. The digital image formingapparatus according to claim 1, further comprising a detection means fordetecting an environmental factor, wherein said discriminating meanssets the ratio in accordance to the image type determined by saiddiscriminating means and to the environmental factor detected by saiddetection means.
 11. The digital image forming apparatus according toclaim 10, wherein the environmental factor comprises humidity.
 12. Thedigital image forming apparatus according to claim 1, further comprisinga detector for detecting a surface potential of said photoconductor,wherein said discriminating means sets the ratio in accordance to theimage type determined by said discriminating means and to the surfacepotential detected by said detector.
 13. The digital image formingapparatus according to claim 12, wherein said detector detects thesurface potentials in a plurality of predetermined exposure conditions.14. The digital image forming apparatus according to claim 1, furthercomprising data means for providing image data according to the signalsoutputted by said image sensor; andwherein said discriminating meansreceives said image data from said data means and processes the thusreceived image data to determine an image type for the original image.15. The digital image forming apparatus according to claim 14, whereinsaid discriminating means comprising a distribution detection means fordetecting a distribution of the image data for at least a portion of theoriginal image.
 16. The digital image forming apparatus according toclaim 14, wherein said discriminating means comprises a distributiondetection means for obtaining a frequency distribution on brightnessdata derived from the image data for at least a portion of the originalimage.
 17. The digital image forming apparatus according to claim 14,wherein said discriminating means includes a distribution detectionmeans which detects a distribution of image data for each of a pluralityof regions of the original image and determines an image type for eachof the plurality of regions.
 18. A digital image forming apparatuscomprising:an image reader which reads an original image and outputssignals according to the original image; a photoconductor; a scannerwhich scans a surface of said photoconductor with light so as to form anelectrostatic latent image; an exposure control means for controlling anintensity of the light generated by said scanner according to signalsreceived from said image reader; a designating means for determining animage type of the original image; and a light-emission control means forinhibiting said scanner to expose said photoconductor for apredetermined time period every time a predetermined number of dots ofimage data is exposed so that non-light emitting interval is formedbetween light-emitting periods when said designating means determinesthat the original image is a multi-level image.
 19. The digital imageforming apparatus according to claim 18, wherein said exposure controlmeans decreases the intensity of the light when said designating meansdetermines that the original image is a multi-level image.
 20. Thedigital image forming apparatus according to claim 18, furthercomprising data means for providing image data according to the signalsoutputted by said image reader; andwherein said designating meansreceives said image data from said data means and processes the thusreceived image data to determine an image type of the original image.21. A method for forming a digital image comprising the stepsof:receiving, with an image sensor, light from an original image andoutputting signals with respect to the original image; exposing asurface of a photoconductor with light emitted from a light source so asto form an electrostatic latent image on the surface of thephotoconductor; controlling an intensity of the light emitted from saidlight source according to the outputted signals; inhibiting the exposingof the surface every predetermined period so that a non-light-emittingtime interval is formed between light-emitting periods; determining animage type of the original image; and changing a ratio of thelight-emitting period to the interval in accordance to the image typedetermined in said step of determining.